Rainwater harvesting, the practice of collecting runoff from rooftops for use in gardening, is an environmentally sound way to conserve municipal water. While many gardeners prefer rainwater due to its natural composition, its safety is not guaranteed. Whether the collected water is safe depends entirely on the materials used in the catchment system and the surrounding environment. The primary concerns involve chemical compounds leaching from the roof structure and pollutants deposited from the atmosphere.
Contaminants from Roofing Materials
The composition of your roof is the primary source of chemical contamination affecting collected rainwater quality. Common asphalt shingles can leach dissolved organic carbon (DOC) and petrochemical compounds into the runoff. These shingles also contain fine granules and heavy metals, such as lead, which wash off and accumulate in the collection tank. The risk from these substances is higher when the water is used on edible plants or in sensitive systems like hydroponics.
Metal roofing materials, particularly copper and galvanized steel, present a significant risk of heavy metal contamination. Copper flashing installed to inhibit moss growth can release copper ions that are toxic to many plants, especially when concentrated. Galvanized metal, coated with zinc for corrosion protection, releases zinc into the water, which can be detrimental to plant health in high doses. Metal leaching is accelerated when the rainwater is naturally acidic, dissolving the metals more readily.
Treated wood shakes, though less common today, can release chemical preservatives into the collected water. Older treatments often included Chromated Copper Arsenate (CCA), containing copper and arsenic. Even modern wood preservatives, which rely heavily on copper compounds, can contribute these substances to the runoff. Using water collected from a roof treated with harsh chemicals or moss killers poses a direct risk of introducing toxins that plants can absorb.
Environmental and Biological Risks
The roof surface acts as a collection plate for contaminants deposited from the air and by wildlife. Airborne pollutants, such as particulate matter from vehicle exhaust and industrial fallout, settle on the roof between rain events. These substances include polycyclic aromatic hydrocarbons (PAHs) and heavy metals, which are concentrated in the first moments of a rainstorm. Rainwater is often mildly acidic due to atmospheric nitrogen and sulfur oxides, and this acidity increases the solubility of any metals it touches.
Biological contamination is a substantial concern, primarily caused by bird and rodent droppings on the roof and in the gutters. Fecal matter introduces pathogenic microorganisms, including bacteria like E. coli, viruses, and parasites. Although plants do not absorb these pathogens, the risk lies in using the water on edible crops where pathogens can remain on the surface of leaves or fruit. General debris like leaves, dust, and pollen also accumulate, contributing organic matter that fosters the growth of bacteria and algae in the storage container.
Essential Water Quality Factors for Plants
Despite contamination risks, rainwater is highly beneficial for plants due to its inherent chemical properties compared to typical municipal sources. Rainwater is naturally soft, meaning it is low in dissolved mineral salts like calcium and magnesium, which can build up in the soil. This low mineral content prevents salt toxicity and mineral crusting, especially around the roots of potted plants. It also helps maintain good soil structure, allowing for better water infiltration and aeration.
The slightly acidic nature of rainwater is a considerable advantage for plant health, typically ranging between pH 5.5 and 6.5. This range is optimal for maximizing the availability and uptake of essential nutrients, such as iron and nitrogen, by plant roots. Conversely, municipal water is often treated to be slightly alkaline to prevent pipe corrosion, but this higher pH can inhibit nutrient absorption in garden soil. Furthermore, rainwater lacks the chlorine, chloramine, or fluoride found in many tap water systems, which can be detrimental to sensitive plants and lead to leaf burn or discoloration.
Safe Collection and Treatment Methods
To mitigate the risks associated with roof runoff, a properly designed collection system employs several layers of pre-filtration. The most effective step is using a “first flush” diverter, a mechanism that automatically routes the initial flow of water away from the storage tank. This initial runoff contains the highest concentration of biological and atmospheric debris settled on the roof. After this initial volume is diverted, a buoyant ball seals the diverter, allowing cleaner subsequent water to flow into the main tank.
System Components
Additional physical pre-filtration should be implemented using gutter screens and downspout leaf guards to block large organic debris like leaves and twigs. The storage tank itself should be opaque and composed of food-grade plastic or lined metal. This prevents light penetration, which inhibits the growth of algae and other microorganisms.
Application Safety
When applying the collected water, minimize the risk of pathogen transfer by watering the soil directly rather than spraying the foliage, especially for leafy greens or root vegetables. Employing drip irrigation or a soaker hose system is the safest application method. Any harvested produce should always be washed thoroughly with potable water before consumption.